An outboard marine engine is typically covered by an engine cover so that splashes and water drops that may be included in the intake air may be effectively removed before they reach the engine. To achieve such a goal, it has been proposed, for instance in Japanese utility model publication (kokai) No. 3-51700, to provide a highly tortuous air intake passage. It has also been proposed to provide an upwardly extending cylindrical duct in the manner of a smoke stack.
However, provision of such a vertically projecting duct unacceptably increases the vertical dimension of the engine assembly. Providing such a duct inside the engine cover inevitably leads to an interference with the engine main body, and even more undesirable problems will be created. For one thing, the internal structure of the engine cover becomes unacceptably complex, not to mention the overall increase in the size of the outer profile. When the intake passage is made tortuous so that water droplets may be separated from the intake air as it passes through the intake passage, it become extremely difficult to change the design of the engine cover afterwards. It is often necessary to make small modifications to the engine cover after it is initially designed. For instance, when the passage diameter is required to be changed, when the performance of the water droplet separation has to be improved, or when the noise of the intake air flow is required to be controlled, the design of the entire engine cover having a relatively large size is required to be changed, and such a need resulted in a substantial increase in the manufacturing cost.